Induction furnaces including a graphite concentrator are used in the field of optical fibers in particular. Optical fibers are fabricated from preforms which are in the form of cylindrical rods the fabrication of which necessitates a shrinking operation. In this operation, a primary preform, which is a hollow silica rod, is heated to a high temperature in a furnace so as to be converted into a solid rod after the diameter of the preform has shrunk as a result of having been subjected to pasty fusion inside the furnace.
During an operation to shrink a primary preform, either the preform moves relative to the furnace or the furnace moves in translation relative to the preform. This translation movement is effected at speeds that can be up to 500 mm/min, which makes it difficult to control the internal atmosphere, mainly because of the effect associated with the movement of the material within the enclosure of the furnace. However, entry of air and therefore of oxygen into the enclosure, the principle consequence of which would be combustion of the graphite and reduction of the service life of the graphite concentrator, must be prevented.
One solution described in the article "R.F. Induction furnace for silica-fibre drawing" published in Electronics Letters 1976, vol. 12, is to use a continuous flow of argon in the furnace enclosure. However, this solution is used in drawing the preform into an optical fiber where the speed of movement of the fiber relative to the furnace is very low (a few mm/minute), and would not be satisfactory in the shrinking operation mentioned hereinabove.